1. Field of the Invention
The present invention relates to a liquid-absorbent sheet that can rapidly absorb and diffuse a liquid excreted from the human body such as urine or menstrual blood, the sheet having a structure for holding the liquid, and a method for producing the same. In particular, the present invention relates to a liquid-absorbent sheet available for paper diapers, sanitary napkins, or the like, and a method for producing the same. In more particular terms, the present invention relates to a liquid-absorbent sheet that is thin and has a high absorbent rate with less wet-back so as to effectively absorb and hold a liquid excreted from the human body.
2. Description of the Related Art
Various absorbent articles that absorb body fluid such as urine or menstrual blood have been known. The absorbent articles usually include a liquid-permeable top sheet, a liquid-impermeable back sheet, and an absorbent body disposed between the top sheet and the back sheet. Such absorbent articles are typified by disposable paper diapers and sanitary napkins and required to have a thin and lightweight shape and to have capabilities for rapidly absorbing and holding the body fluid such as urine or menstrual blood as much as possible without side leakage. Also, in recent years, such absorbent articles have been made compact, and therefore more stringently demanded to be thin. Moreover, such absorbent articles have been demanded to absorb the body fluid quickly with little return of the body fluid to the surface thereof at the skin side, and to be excellent in quality stability.
Such absorbent articles generally include an absorbent body in which a super absorbent polymer (hereinafter, abbreviated as SAP) is mixed in a pulverized pulp. Although the content of the SAP is generally increased so as to improve the absorbent rate, such an increase makes the absorbent body thicker, and thereby it becomes difficult to make the absorbent article compact. Moreover, although the SAP is usually added during pulverizing a sheet pulp to be mixed and pulverized with the pulp so as to reduce unevenness of the quality of obtained absorbent bodies, there is a case in which a part of the SAP is lost during the pulverizing step, and thereby the obtained absorbent bodies do not have sufficient absorbabilities. Moreover, when the thickness of the absorbent body is reduced by pressurization in addition to the above step so as to make the absorbent article thinner, the density of the SAP is increased, which suppresses the permeation of an absorbed liquid, and gaps of the SAP are reduced, which may make the liquid absorbability insufficient.
In order to solve the above problems, Patent Document 1 (Japanese Patent Application, First Publication No. 2002-35036) has proposed that perforating holes be formed on the surface of an absorbent body so as to improve the absorbability thereof. By forming such perforating holes, the absorbed liquid transfers to the back-sheet side at an initial stage, and so the absorbent rate is improved. Moreover, the absorbed liquid spreads around the perforating holes, and so the diffusion properties are also improved. However, when the perforating holes are formed, the qualities of obtained absorbent bodies vary widely, and a wet-back phenomenon in which the absorbed liquid returns through the perforating holes is caused.
On the other hand, Patent Document 2 (Japanese Patent Application, First Publication No. H 8-107910) has proposed an absorbent article in which the SAP is arranged to form parallel stripes. When the SAP is arranged to form parallel stripes, the diffusion properties and absorbability of the absorbent article are significantly improved, and so the absorbed liquid is rapidly absorbed and diffused. The reason for this is that vacant spaces produced in such an arrangement enable the SAP absorbing the liquid to swell. However, since the SAP is unevenly distributed, and so the absorbent article becomes thick, such an arrangement is not suitable for thinning the article.
Also, Patent Document 3 (Published Japanese translation No. 2003-508647 of PCT International Publication) discloses a method for producing an absorbent material that is used to form an absorbent article, the method aiming at stabilization of the liquid-absorbability of the absorbent article and reduction of the thickness thereof. In the method, the SAP is evenly dispersed in a pulp, and so the stability of the absorbability is excellent. Also, a press treatment is carried out so as to prepare the absorbent material as a thin sheet, and so the absorbent material is suitably available for reducing the thickness of the absorbent article. However, since the press treatment is carried out while adding water to the sheet, the SAP locating at the surface of the sheet is swollen, which may reduce the vacant space, and so the absorbent rate thereof may decrease.